This invention relates to a controller for an acoustic echo suppressor and a method of controlling an acoustic echo suppressor.
In telephony, an echo is a reflection of the voice signal. It is a delayed copy of the original. An example scenario is illustrated in FIG. 1, which shows a signal being captured by a far-end microphone and output by a near-end loudspeaker. The echo is a consequence of acoustic coupling between the loudspeaker and the near-end microphone; the near-end microphone captures the signal originating from its own loudspeaker in addition to the voice of the near-end speaker and any background noise. The result is an echo at the far-end loudspeaker. Echo cancellation is an important feature of telephony. Hands-free devices and teleconferencing, in particular, require echo cancellation that can adapt to environments having a wide range of acoustic characteristics.
Echo cancellers typically synthesise an estimate of the echo from the far-end voice signal. The estimated echo is then subtracted from the microphone signal. This technique requires adaptive signal processing to generate a signal accurate enough to cancel the echo effectively. An adaptive filter is often used to model the environment's acoustic impulse response.
An acoustic echo canceller and adaptive filter are described in International Patent Application WO 2012/158163, incorporated by reference herein in its entirety. The acoustic echo canceller described therein uses a non-linear processor operating in the frequency domain to determine suppression factors for each of a plurality of frequency bands. The echo canceller uses the suppression factors to control the removal of echo from a near-end audio signal. However, even though the echo canceller works reasonably well in high echo return loss scenarios, it suffers from poor performance during low echo return loss scenarios (i.e. high ratios of echo to near-end signal). Additionally, the computational cost of coherence measures between signals in the frequency domain is high.
Even with high performance adaptive filters it is not always possible for an echo canceller to remove all echoes from a signal, and the echo cancelled signal from an echo canceller will often include residual echo of the far-end voice signal. This is because the echo estimate generated by an adaptive filter will not always precisely match the true echo in the microphone signal. There can be several reasons for this, including loss of convergence of the adaptive filter due to changes in echo path and as a result of freezing the adaptive filter during near-end speech to avoid wide divergence of the filter.
In order to address the problem of residual echo in a microphone signal following echo cancellation, an echo suppressor can be used to remove the residual echo by replacing or masking the microphone signal when residual echo is present. To ensure that an echo suppressor is enabled only at appropriate moments, echo suppressors are typically controlled according to the presence of near-end speech. This is with the aim of avoiding the introduction of artefacts into the microphone signal or otherwise interfering with near-end speech carried in the microphone signal. U.S. Pat. Nos. 6,507,653 and 6,532,289 describe detectors for identifying near-end speech and controlling an echo suppressor according to whether or not near-end speech is identified. However, using conventional near-end speech detectors to control an echo suppressor can lead to clipping of double talk during periods of high echo relative to the near-end speech present in a microphone signal.